The proliferation of fiber-optic cables and the ever increasing demand for new broadband services is moving future telecommunication networks toward all-optical networks. By design, all-optical networks perform key signal processing such as switching, multiplexing, demultiplexing, amplification, and correlation, with optical systems and avoid electrical-to-optical and optical-to-electrical conversions. Optical systems or optical signal processing should alleviate the predicted bottleneck that could occur with complex high-speed electronic switches, multiplexers, demultiplexers, and so forth, because all-optical techniques are potentially much faster than electrical signal processing. Several new classes of optical networks are emerging. In particular, code division multiple access (CDMA) networks using optical signal processing technique have been recently introduced. For example, the special issue on "Optical Multiaccess", as published in the IEEE Network Magazine, vol. 3, no. 2, March 1989 provides an overview of this emerging field.
In a typical CDMA system, multiaccess is achieved by assigning different, minimally interfering code sequences to different user pairs. Users then communicate by imprinting their message bits upon their own unique code, which they transmit asynchronously (with respect to the other transmitters) over a common channel. A matched filter at the receiver end ensures that message bits are detected only when they are imprinted on the proper code sequence. This approach to multiaccess allows transmission without delay and handles multiaccess interference as an integral part of the scheme.
Processing gain (PG) for CDMA techniques is a critical parameter which may be used to judge the relative merits of CDMA systems. Processing gain is broadly given by the ratio of total transmitted bandwidth to information bandwidth of a transmitter. The value of PG establishes an upper bound on the number of users/transmitters that can be simultaneously active on a given CDMA system. Presently known CDMA techniques such as spread-spectrum and spread-time (which will be discussed in detail below) can only incrementally increase the PG since the total transmitted bandwidth is usually fixed, implying that the information bandwidth must be decreased in order to increase the PG. A large reduction in information bandwidth is difficult to achieve for arbitrary information sources.
In addition, with conventional CDMA techniques all transmitters have essentially the same signal format and data rate. This precludes a mixed multiuser environment wherein it is desired to transmit analog voice, low rate data, a TV signal, and so forth simultaneously over the CDMA system.